causes at work.

The shore shelf stands about five feet above low water. A small island in this bay is well named the "Old

66 THE OLD HAT," BAY OF ISLANDS.

Hat," the platform encircling it, as shown in the above figure, forming a broad brim to a rude conical crown. The water, in these cases, has worn away the cliffs, leaving the basement untouched.

A surging wave, as it comes upon a coast, gradually rears itself on the shallowing shores; finally, the waters at top, through their greater velocity, plunge with violence upon the barrier before it. The force of the ocean's surges is therefore mostly confined to their summit waters, which add weight to superior velocity, and drive violently upon whatever obstacle is presented. The lower waters of the surge advance steadily but more slowly, owing to the retarding friction of the bottom; the motion they have is directly forward, and thus they act with little mechanical advantage; moreover, they gradually swell over the shores, and receive, in part, the force of the upper waters. The wave, after breaking, sweeps up the shore till it gradually dies away. Degradation from this source is consequently most active where the upper or plunging portion of the breaker strikes.

But, further, we observe that at low tide the sea is comparatively quiet; it is during the influx and efflux that the surges are heaviest. The action commences after the rise, is strongest from half to three-fourths tide, and then diminishes again near high tide. Moreover, the plunging part of the wave is raised considerably above the general level of the water. From these considerations, it is apparent that the line of greatest wave-action, must be above low water level. Let us suppose a tide of three feet, in which the action would probably be strongest when the tide had risen two feet out of the three; and let the height of the advancing surge be four feet-the wave, at the time of striking, would stand, with its summit, three feet above high tide level; and from this height would plunge obliquely downward against the rock, or any obstacle before it. It is obvious, that under such circumstances, the greatest force would be felt, not far from the line of high tide, or between that line and three feet above it. In regions where the tide is higher than just supposed, as six feet for example, the same height of wave would give

nearly the same height to the line of wave action, as compared with high tide level. Under the influence of heavier waves, such as are common during storms, the line of wave-action would be at a still higher elevation, as may be readily estimated by the reader.

Besides a line of the greatest wave-action, we may also dis tinguish a height where this action is entirely null; and it is evident, from facts already stated, that the point will be found somewhat above low tide level. The lower waters of the surge, instead of causing degradation, are accumulative in their ordinary action, when the material exposed to them is movable: they are constantly piling up, while the upper waters are rending and preparing material to be carried off. The height at which these two operations balance one another will be the height, therefore, of the line of no degradation. As the sea at low tide is mostly quiet, and the lower of the surging waters swell on to receive the upper and parry the blow, and moreover, there is next a return current outward,-we should infer that the line would be situated more or less above low tide, according to the height of the tide, and the surges accompanying it. We are not left to conjecture on this point; for the examples presented by the shores of New Holland and New Zealand afford definite facts. Degradation has there taken place sufficient to carry off cliffs of rock, of great extent; yet below a certain level, the sea has had little or no effect. This height, at New Holland, is three feet above ordinary low tide, and at New Zealand, about five feet. With regard to the height varying with the tides, we observe that in the Paumotus, where the water rises but two or three feet, the platform is seldom over four to six inches above low tide, which is proportionally less than at New Holland and New Zealand, where the tide is six and eight feet. From these observations, it appears that the height of no wave-action, as regards the degrada tion of a coast under ordinary seas, is situated near one-fifth tide, in the Paumotus, and above half tide at New Zealand, showing a great difference between the effect of the comparatively quiet surges of the middle Pacific, and the more violent of New Zea land. Within the Bay of Islands, where the sea has not its full force, the platform, as around the "Old Hat," is but little above low water level. The exact relation of the height of the platform to the height and force of the tides remains to be determined more accurately by observation. While, therefore, the height of the shore platform depends on the tides, and the usual strength of the waves, the breadth of it will be determined by the same causes in connection with the nature of the rockmaterial.*

* On basaltic shores it is not usual to find a shore platform, as the rock scarcely ndergoes any degradation, except from the most violent seas; such coasts are con

It is apparent that one single principle meets all the various The rocky platform of some sea-shores, the low tide. sand-spit on others, and the coral-reef platform of others, require but one explanation. The material of the coral platform is piled up by the advancing surges, and cemented through the infiltrating waters. These surges, advancing towards the edge of the shelf, swell over it before breaking, and thus throw a protection about the exposed rocks; and as the tide rises, this protection is complete. They move on, sweeping over the shelf, but only clear it of sand and fragments, which they bear to the beach.

The isolated blocks in the Paumotus which stand on the platform, attached to it below, are generally most worn one or two feet above high tide level, a fact which corresponds with the statement in a preceding paragraph with regard to the height of the greatest wave-action.

In addition to this ordinary wave-action, there are also more violent effects from storms; and these are observed alike on the Australian shores referred to, and on those of coral islands. The waters, moving through greater depths, and driving on with increased velocity up the shallowing shores among cavities or under shelving layers, break and lift the rocks of the edge of the platform, and throw them on the reef. From the observations of Mr. Stevenson, cited in a note to a preceding page, it appears that the force of the waves during the summer and winter months differs at Skerryvore more than 1200 pounds to the square foot, in the former it averaging but 636 pounds, and in the latter 2086 pounds, while in storms it was at times equivalent to 6083 pounds. The seasons are not as unlike in the tropical part of the Pacific. Still there must be a marked difference between the ordinary seas and those during stormy weather. We have therefore no difficulty in comprehending how the ordinary waveaction should build up and keep entire the shore platform, while the more agitated seas may tear up parts of the structure formed, and bear them on to the higher parts of the island. Still more violent in action are the great earthquake-waves, which move through the very depths of the ocean.

These principles offer an explanation also of the general fact that the windward reef is the highest. The ordinary seas, both on the leeward and windward sides, are sufficient for producing coral debris and building up the reef, and in this work the two sides may go on with almost equal rate of progress: consequently we may often find no very great difference in the width of the

sequently often covered with large fragments of the basaltic rocks. But on sandstone shores, this gradual action keeps the platform of nearly uniform breadth. Moreover, any uptorn masses thrown upon it, are soon destroyed by the same action, and carried off; and thus the platform is kept nearly clean of debris, even to the base of the cliff.

leeward and windward reefs, especially as the wind for some parts of the year has a course opposite to its usual direction. But seldom, except on the side to windward, is a sufficient force brought to bear upon the edge of the platform, to detach and uplift the larger coral blocks. The distance to which the waves may roll on without becoming too much weakened for the transportation of uptorn blocks, will determine the outline of the forming land. With proper data as to the force of the waves, the tides, and the soundings around, the extent of the shore platform might be made a subject of calculation.

The effect of a windward reef in diminishing the force of the sea is sometimes shown in the influence of one island on another. A striking instance of this is presented by the northernmost of the Tarawan Islands. All the islands of this group are well wooded to windward-the side fronting east, between north and south. But the north side of Tari-tari is nothing but a bare reef, through a distance of twenty miles, although the southeast reef is a continuous line of verdure. The small island of Makin, just north of Tari-tari, is the break water which has protected the reef referred to from the heavier seas.

Coral island accumulations have one advantage over all other shore deposits, owing to the ready agglutination of calcareous grains, as explained on a following page. It has been stated that coral sandrocks are forming along the beaches, while the reefrock is consolidating in the water. A defence of rock against encroachment is thus produced, and is in continual progress. Moreover, the structure built amid the waves will necessarily have the form and condition best fitted for withstanding their ac tion. The little islet of an atoll is therefore more enduring than hills of harder basaltic rocks. Reefs of zoophytic growth but "mock the leaping billows," while other lands of the same height gradually yield to the assaults of the ocean. There are cases, however, of wear from the sea, owing to some change of condition in the island, or in the currents about it, in consequence of which, parts once built up are again carried off. Moreover, those devastating seas which overleap the whole land may occa sion unusual degradation from some parts. Yet these islets have within themselves the source of their own repair, and are secure from all serious injury.

The lagoons in coral islands are constantly receiving more or less debris from the reefs; and patches of growing coral within also tend to fill them up. But the effect is slow in its progress, and none but islands of small size, as before stated, show any approximation to an obliteration of the lagoon.

ART. XXXIV.-Optical and Blowpipe Examination of the supposed Chlorite of Chester County, Pa.; by W. P. BLAKE.

Read before the American Association for the Advancement of Science, at Albany, August, 1851.

IN September, 1850, Prof. B. Silliman, Jr., handed me a specimen of a beautiful green foliated mineral for optical examination; it was unexpectedly found to be biaxial; but as the locality of the specimen was not known, no further examination than the measurement of the angles was made at that time. In May, of this year, I received from Prof. J. D. Dana specimens of the hitherto supposed chlorite, of Chester Co., Pa., which I examined by polarized light, and obtained results so similar to those obtained with the specimen first referred to, as to leave no doubt of its being from the same locality.*

The mineral occurs three miles south of West Chester, in serpentine associated with magnesite, and is found in plates of irregular outline, sometimes three inches broad, and in triangular plates and tabular masses, one of which is represented in the annexed figure. These plates are equilateral triangles; and they much resemble the triangular cleavage specimens of the micas from Greenwood furnace and Monroe, N. Y. The cleavage is perfect, parallel with the broad faces of these crystals, but is not so perfect as in mica, and the laminæ are more brittle. The lamina are flexible and elastic, but less elastic than mica. Color, beautiful

emerald green. Hardness of cleavage surface, 2 to 2.25, scale of Mohs. Specific gravity 2.714, which is perhaps too low, as no specimen could be obtained perfectly free from air.

Optically it is biaxial, with a high angle, and the following are the results obtained:

Specimen a, examined in September, plate one decimetre long and six centimetres broad, with an irregular outline.

Specimen b, a triangular plate measuring one and one-fourth inches along each side, examined in May.

Apparent angle between the optic axes in a, 84°30′ mean of nine measurements.

Apparent angle in b, 85°59′ mean of five measurements.

The plane of the axes is perpendicular to the cleavage surface and at right angles with the base of the triangle, as indicated by the arrow in the figure. I was also able to obtain evidences of

* Prof. Dana received his specimen from Thos. F. Seal, of Philadelphia.